Wire Boards and Anchoring Elements

To guide and secure the 3520 wires on an APA, stacks of custom FR4 circuit boards attach to the outside edges of the frame, as shown in the engineering drawings in Figure 2.11. There are

204 wire boards on each APA and 337 total circuit boards, where this number includes the wire

boards, cover boards, capacitive-resistive (CR) boards, G-layer bias boards, adapter boards, and one SHV board.

Figure 2.11: Engineering drawings that illustrate the layering of the wire carrier boards that are secured

along the perimeter of the APA steel frames. Left: The full set ofV-layer boards. Right: Detail showing

the full stack of four boards at the head end of the APA.

2.2.5.1 Head Electronics Boards

All APA wires are terminated on wire boards that are stacked along the electronics end of the APA frame. The board stack at the head end is shown in Figure 2.11. Attachment of the wire

boards begins with theX-plane (lowest). Once the X-plane wires are strung on both sides of the

APA frame, they are soldered and epoxied to their wire boards and trimmed. The remaining wire board layers are attached as each previous layer of wires are placed. The wire plane spacing of 4.75 mm is set by the thickness of these wire boards.

Mill-Max 2 pins and sockets provide electrical connections between circuit boards within a stack.

They are pressed into the circuit boards and are not repairable if damaged. To minimize the possibility of damaged pins, the boards are designed so that the first wire board attached to the frame has only sockets. All boards attached subsequently contain pins that plug into previously mounted boards. This process eliminates exposure of any pins to possible damage during winding, soldering, or trimming processes.

The X, U and V layers of wires are connected to the CE (housed in boxes mounted on the APA)

either directly or through DC-blocking capacitors. Ten stacks of wire boards are installed across

the width of each side along the head of the APA. TheX-layer board in each stack has room for 48

wires, the V-layer has 40 wires, the U-layer 40 wires and the G-layer 48 wires. Each board stack,

therefore, has 176 wires but only 128 signal channels since the G wires are not read out. With a

total of 20 stacks per APA, this results in 2560 signal channels per APA and a total of 3520 wires starting at the top of the APA and ending at the bottom. Many of the capacitors and resistors that in principle could be on these wire boards are instead placed on the attached CR boards (see next section) to improve their accessibility in case of component failure. Figure 2.12 depicts the

connections between the different elements of the APA electrical circuit at the head end of the frame.

Figure 2.12: The wire board stack at the head end of an APA and the connection to the CE. The set of wire boards within a stack can be seen on both sides of the APA, with the CR board extending further to the right to provide a connection to the CE.

2.2.5.2 CR Boards

The capacitive-resistive (CR) boards carry a bias resistor and a DC-blocking capacitor for each

wire in the X and U-planes. These boards are attached to the board stacks after fabrication of

all wire planes. Electrical connections to the board stack are made though Mill-Max pins that plug into the wire boards. Connections from the CR boards to the CE are made through a pair of

96-pin Samtec 3 connectors.

Surface-mount bias resistors on the CR boards have resistance of 50 MW and are constructed with

a thick film on a ceramic substrate. Rated for 2.0 kV operation, the resistors measure 3.0×6.1 mm

(0.12×0.24 in). The selected DC-blocking capacitors have capacitance of 3.9 nF and are rated

for 2.0 kV operation. Measuring 5.6×6.4 mm (0.22×0.25 in) across and 2.5 mm (0.10 in) high, the

capacitors feature flexible terminals to comply with PC board expansion and contraction. They are designed to withstand 1000 thermal cycles between the extremes of the operating temperature range. Tolerance is also 5 %.

In addition to the bias and DC-blocking capacitors for all X and U-plane wires, the CR boards

include two R-C filters for the bias voltages. The resistors are of the same type used for wire biasing except with a resistance of 2 MΩ. Wire plane bias filter capacitors are 39 nF, consisting of ten 3.9 nF surface-mount capacitors connected in parallel. They are the same capacitors as those used for DC blocking.

The selected capacitors were designed by the manufacturer to withstand repeated temperature excursions over a wide range. Their mechanically compliant terminal structure accommodates CTE mismatches. The resistors employ a thick-film technology that is also tolerant of wide temperature excursions. Capacitors and resistors were qualified for ProtoDUNE-SP by subjecting samples to

repeated testing at room temperature and at −190◦C. Performance criteria were measured across

five thermal cycles, and no measurable changes were observed. During the production of 140 CR boards, more than 10,000 units of each component were tested at room temperature, at LAr temperature, and again at room temperature. No failures or measurable changes in performance were observed.

2.2.5.3 Side and Foot Boards

The boards along the sides and foot of the APA have notches, pins, and other location features to hold the wires in the correct position as they wrap around the edge from one side of the APA to the other.

Figure 2.13: Side boards with traces that connect wires around openings. The wires are wound straight over the openings, then soldered to pads at the ends of the traces, then the wire sections between the pads are trimmed away.

A number of hole or slot features are needed in the edge boards to provide access to the underlying frame (see Figure 2.13 for examples). In order that these openings not be covered by wires, the sections of wire that would go over the openings are replaced by traces on the boards. After the wires are wrapped, the wires over the opening are soldered to pads at the ends of the traces and the section of wire between the pads is snipped out. These traces are easily and economically added to the boards by the many commercial fabricators who make circuit boards.

The placement of the angled wires are fixed by teeth that are part of an injected molded strip that is glued to the edge of the FR4 boards. The polymer used for the strips is Vectra e130i (a trade name for 30% glass filled liquid crystal polymer, or LCP). It retains its strength at cryogenic temperature and has a CTE similar enough to FR4 that differential contraction is not a problem. The wires make a partial wrap around the pin as they change direction from the face of the APA to the edge.

2.2.5.4 Support Combs

Support combs are glued at four points along each side of the APA, along the four cross beams. These combs maintain the wire and plane spacing along the length of the APA. A dedicated jig is used to install the combs and provides the alignment and the pressure to allow the glue to dry. The glue used is the Gray epoxy 2216 described below. An eight-hour cure time is required after comb installation on each side of the APA before the jig can be removed and production can continue. Figure 2.14 shows a detail of the wire support combs on a ProtoDUNE-SP APA.

Figure 2.14: Left: APA corner where end boards meet side boards. The injection molded teeth that

guide the U and V wires around the edge are visible at the bottom. Right: The wire support combs.

2.2.5.5 Solder and Epoxy

The ends of the wires are soldered to pads on the edges of the wire boards. Solder provides both an electrical connection and a physical anchor to the wire pads. A 62% tin, 36% lead, and 2% silver solder was chosen. A eutectic mix (63/37) is the best of the straight tin-lead solders but the 2% added silver gives better creep resistance.

Once a wire layer is complete, the next layer of boards is glued on, this glue providing an additional

physical anchor. Gray epoxy 2216 by 3M4 is used for the glue. It is strong, widely used (therefore

much data is available), and it retains good properties at cryogenic temperatures.